12.9.3 Full-Text Searches with Query Expansion

Full-text search supports query expansion (and in particular,
its variant “blind query expansion”). This is
generally useful when a search phrase is too short, which often
means that the user is relying on implied knowledge that the
full-text search engine lacks. For example, a user searching for
“database” may really mean that
“MySQL”, “Oracle”, “DB2”,
and “RDBMS” all are phrases that should match
“databases” and should be returned, too. This is
implied knowledge.

Blind query expansion (also known as automatic relevance
feedback) is enabled by adding WITH QUERY
EXPANSION or IN NATURAL LANGUAGE MODE WITH
QUERY EXPANSION following the search phrase. It works
by performing the search twice, where the search phrase for the
second search is the original search phrase concatenated with
the few most highly relevant documents from the first search.
Thus, if one of these documents contains the word
“databases” and the word “MySQL”, the
second search finds the documents that contain the word
“MySQL” even if they do not contain the word
“database”. The following example shows this
difference:

Another example could be searching for books by Georges Simenon
about Maigret, when a user is not sure how to spell
“Maigret”. A search for “Megre and the
reluctant witnesses” finds only “Maigret and the
Reluctant Witnesses” without query expansion. A search
with query expansion finds all books with the word
“Maigret” on the second pass.

Note

Because blind query expansion tends to increase noise
significantly by returning nonrelevant documents, it is
meaningful to use only when a search phrase is rather short.